This invention relates broadly to the art of valve actuation, and more specifically to power valve actuation.
Large valves, such as knife gate valves used to control the flow of coal slurry, sewage, water, vacuums, chemicals, and the like, through large pipes, can often be difficult to operate. For this reason, it has been common practice to provide gate-blade actuators having manually-driven wheels for engaging gate blades via high-mechanical-advantage threaded shafts. However, as can be imagined, such activation of large valves is time consuming, labor intensive, strenuous, and requires a certain amount of strength. Thus, it is an object of this invention to provide a valve actuator which operates valves relatively quickly with expenditures of very little effort, requiring virtually no strength.
To overcome the above mentioned problems, a type of pneumatic valve actuators for knife-gate valves has been developed which basically comprises a cylinder with a piston therein having a piston shaft extending out an end of the cylinder where it is attached to a knife-gate valve. When it is desired to actuate the knife-gate valve, a pilot valve is moved to supply pressurized air to an appropriate end of the cylinder for driving the piston in the cylinder and thereby driving the knife-gate valve. Use of pressurized air has the benefit that exhausted air can be released from the cylinder to atmosphere so that a return air hose is not required. Also, with pressurized air one need not be unduly concerned about leaking fluid. Also, lower pressures are normally used than are used with hydraulic systems. However, use of pressurized air has the disadvantage that it requires greater diameter cylinders and pistons to achieve a required thrust than would a hydraulic system since lower pressures are normally used and air is compressible. Also, when pneumatic-valve actuation systems are used, it is usually more economical and convenient to have a centralized compressor and surge tank, with pneumatic hoses extending therefrom to various valve actuation devices. Such pneumatic hoses are often cumbersome and inconvenient and must be protected. Also, in cold weather air lines with water from condensation will freeze and the air lines will become stopped up with ice. This is common in the mining industry. Similarly, it is usually not economical or convenient to invest in such a pneumatic power system for a small number of valves. Thus, it is an object of this invention to provide a power valve-actuation system which does not use high pressures, does not require long, inconvenient hoses, and which is cost effective and convenient for use with a small number of valves.
It has been suggested to use hydraulic valve actuators for actuating knife-gate valves; however, this is seldom done. A problem with hydraulic valve actuators is that they normally operate at such high pressures, and with such great forces that they can cause damage when they are inadvertently closed on hard objects travelling in fluid lines. Also, extreme care in sizing actuators using high pressure hydraulic fluids is required because a thrust created could exceed design limitations of a valve being actuated. Also, most hydraulic systems are like air system with a central power supply system which runs continuously, which consumes energy, and produces heat (friction in hydraulic lines) that will shorten the life of the system.
Similarly, many power-driven valve actuators of this type often have limit switches which when a valve actuator is switched to close a valve, for example, deenergize the valve actuator once it is sensed that the valve is in a closed position. However, if the valve encounters a hard object and is thereby prevented from reaching a closed position, the limit switch never senses that the valve is closed and the valve actuator continues to try to drive the valve closed with high hydraulic pressure. Not only can such action cause damage to the valve, it can also damage the valve actuator by applying high pressures over long periods of time. Torque or pressure switches can be used to prevent this, however, such switches often malfunction. For this reason, it is an object of this invention to provide a valve-actuator system which when a valve being thereby driven encounters a hard object preventing the valve from closing nevertheless deactivates a valve actuator once it has been given a chance to close the valve. Similar problems can arise when one uses an electro-mechanical valve-actuator system for closing and opening a valve. It is therefore yet another object of this invention to provide a valve-actuator system which deactivates a valve actuator once it has been given an opportunity to move a valve to a desired position.
As mentioned above, some companies have installed pneumatic valve-actuator systems having central compressors and surge tanks supplying pressurized pneumatic fluid (air) to disbursed valve actuators via hoses. These companies have invested a great deal of money in these pneumatic systems and are therefore reluctant to switch to other power systems which might overcome some of the disadvantages of pneumatic power systems. Therefore, it is another object of this invention to provide a valve-actuator system, and a method of its installation such that it can be retrofitted to preexisting pneumatic valve systems with much of the equipment being retained and used in the newly installed power valve-actuator system.
It is also another object of this invention to provide a valve-actuator system and method of use thereof which is relatively inexpensive and convenient.
It is yet another object of this invention to provide a fail safe (fail open or fail closed) valve actuation in case of power failure.